Ultrafaint [C ii] Emission in a Redshift=2 Gravitationally Lensed Metal-poor Dwarf Galaxy

Extreme emission-line galaxies (EELGs) at redshift z = 1-2 provide a unique view of metal-poor, starburst sources that are the likely drivers of the cosmic reionization at z >= 6. However, the molecular gas reservoirs of EELGs-the fuel for their intense star formation-remain beyond the reach of current facilities. We present ALMA [C ii] and PdBI CO(2-1) observations of the z = 1.8, strongly lensed EELG SL2S 0217, a bright Ly alpha emitter with a metallicity 0.05 Z. We obtain a tentative (similar to 3 sigma-4 sigma) detection of the [C ii] line and set an upper limit on the [C ii]/SFR (star-forming rate) ratio of <= 1 x 10(6) L/(M yr(-1)), based on the synthesized images and visibility-plane analysis. The CO(2-1) emission is not detected. Photoionization modeling indicates that up to 80% of the [C ii] emission originates from neutral or molecular gas, although we cannot rule out that the gas is fully ionized. The very faint [C ii] emission is in line with both nearby metal-poor dwarfs and high-redshift Ly alpha emitters, and predictions from hydrodynamical simulations. However, the [C ii] line is 30x fainter than predicted by the De Looze et al. [C ii]-SFR relation for local dwarfs, illustrating the danger of extrapolating locally calibrated relations to high-redshift, metal-poor galaxies.

Automated summary

Observations of the strongly lensed EELG SL2S 0217 at redshift z = 1.8 show tentative detection of the [C ii] line and no detection of CO(2-1) emission. Photoionization modeling suggests that up to 80% of the [C ii] emission may originate from neutral or molecular gas, in line with some predictions.